1. Technical Field
This invention relates generally to a method of in-situ formation of a metal oxide material. In particular to the in-situ forming of ferroelectric or films with a ferroelectric oxide.
2. Background
In order to build an integrated circuit, many active devices need to be fabricated on a single substrate. The current practice in semiconductor manufacturing is to use thin film fabrication techniques. A large variety of materials can be deposited using thin films, including metals, semiconductors, insulators and the like. The semiconductor industry""s continuing drive towards closer and smaller device geometries, has placed an increased demand for cost-effective solutions for the fabrication problems related to the decreasing geometries. Several problems arise, in particular for memory circuits.
One of the most commonly used memory circuits are random access memory (RAM) circuits. RAM is generally used as the basic high speed memory in digital computers. One problem with these memories is that the memories are generally volatile. A memory device is called volatile if the stored data disappears after a power loss or interruption. In many applications, sudden loss of the stored data in these memories is undesirable. A memory device is called nonvolatile if the stored data will not disappear, even after a power break. Ferroelectrics can be used in RAM memory circuits for the insulation or dielectric layer in the data-storage capacitors in memory circuits to achieve nonvolatility.
Ferroelectrics are compatible with conventional wafer processing and memory circuits. A ferroelectric is a dielectric material that exhibits hysteresis such that a large amount of polarization will survive even when the electric field is reduced to zero. This allows a capacitor made with a ferroelectric insulating layer to permanently retain charge after application of a voltage. Because of the molecular structure of a ferroelectric it can retain a charge in two different configurations, giving it essentially binary capabilities desirable for use in RAM memory circuits.
A second problem for RAM circuits, resulting from the ever shrinking size also has to do with the functioning of the capacitor. With a conventional capacitor, when the area is made very small, the capacitor is unable to accumulate enough charge to operate the semiconductor memory circuit correctly. One possible solution to this problem is to use an insulating film with a high dielectric constant. In addition to providing the characteristic of nonvolatility for memory circuits, ferroelectric capacitors also have a comparatively much higher dielectric constant than more conventional silicon dioxide capacitors and can be used to mitigate the charge accumulation problem. Because ferroelectrics have a high dielectric constant much smaller ferroelectric capacitors may be used in place of silicon dioxide capacitors. This allows RAM memory circuits made with ferroelectric capacitors to store a sufficient charge in a smaller area. This is extremely advantageous as active area dimensions evolve towards the submicron level to meet the ever decreasing space requirements in circuit fabrication.
Many ferroelectric materials exhibit the same atomic structure as a regular perovskite crystal. A unit cell of a perovskite crystal has a general chemical formula of ABO3, where A is a large cation and B is a small cation. A perovskite crystal has a central metallic ion that is displaced into one of two positions along the axis of an applied electric field to create an electric dipole. The central ion remains polarized until an electric field is again applied to reverse it. In addition to their ferroelectric properties, the materials with perovskite structure are also known to be useful for their electro-optic properties and superconducting properties.
A variety of techniques have been used for the deposition of ferroelectrics as thin films. Techniques that have been used include wet chemical processes, chemical vapor deposition (CVD) processes and physical vapor deposition (PVD) processes. The wet chemical processes encompass sol-gel processes and metalorganic decomposition.
The PVD processes include, for example, molecular beam epitaxy, electron beam evaporation, ion beam sputtering, radio frequency (rf) diode sputtering, rf magnetron sputtering, dc magnetron sputtering, and laser ablation.
Ishihara, U.S. Pat. No. 5,397,446 describes a method of forming a ferroelectric film that includes forming a first layer by sputtering at a first pressure, and forming a second layer on the first layer by sputtering at a second lower pressure.
Gnadinger, U.S. Pat. No. 5,273,927 teaches a five step process that includes forming a layer of insulative material on a source, followed by the formation of a first conductive layer on top of the insulative layer, then the construction of a ferroelectric layer and a second conductive layer is then formed on top of the ferroelectric layer.
Desu, et al, U.S. Pat. No. 5,431,958 describes a method of forming a ferroelectric lead zirconate titanate film by metal organic CVD. The method teaches the introduction of separate lead, zirconium and titanium precursor compounds and an oxidizing agent into a CVD reactor under specified conditions.
In all of the conventional processes that use the perovskite material as a ferroelectric, the top and bottom plate of the capacitor and the insulating middle layer are fabricated separately and sequentially. The top and bottom plates are made out of conductive material, to function as electrodes; with the insulator or dielectric disposed between the two electrodes. That is to say that a first electrode or bottom plate is fabricated, most commonly by CVD or plasma-assisted deposition, followed by the deposition of the ferroelectric material onto the bottom plate and then the top plate is fabricated on top of the ferroelectric or perovskite dielectric layer.
In addition to the fact that all of the conventional processes for fabricating a ferroelectric capacitor construct each layer separately, the most commonly used processes for making a ferroelectric film are often cumbersome and difficult to integrate with other common fabrication steps used in semiconductor device manufacture. What is still needed is a method to fabricate metal oxide and ferroelectric films and devices that is more efficient and compatible with the overall process of semiconductor fabrication.
The present invention is a method of in-situ formation of metal oxide films and more particularly ferroelectric films. In the case of ferroelectric films the method comprises the steps of: forming a starting layer of metal on top of an oxide layer, wherein the metal for the starting layer is capable of forming a ferroelectric material by reducing the underlying oxide layer. Incorporating into the starting layer at least a second metal capable of being oxidized by the products of the reduction of the oxide layer by the first metal, forming a layer containing a plurality of metals, and heating the plurality of metals layer in the presence of nitrogen.
Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.